The capacity to quickly, reliably and cleanly sever a pipe or well casing deeply within a wellbore is an essential maintenance and salvage operation in the petroleum drilling and exploration industry. Cutting large, 7 inch to 20 inch nominal diameter casing and casing liner is particularly challenging. Generally, the industry relies upon mechanical, chemical or pyrotechnic devices for such cutting. Among the available options, shaped charge (SC) explosive cutters are often the simplest, fastest and least expensive tools for cutting pipe in a well. The devices are typically conveyed into a well for detonation on a wireline or length of coiled tubing.
Typical explosive pipe cutting devices comprise a consolidated wheel of explosive material having a V-groove perimeter similar to a V-belt drive sheave. The surfaces of the circular V-groove are clad with a thin metal liner. Pressed contiguously against the metal liner is a highly explosive material such as HMX, RDX or HNS.
This V-grooved wheel of shaped explosive is aligned coaxially within a housing sub and the sub is disposed internally of the pipe that is to be cut. Accordingly, the plane that includes the circular perimeter of the V-groove apex is substantially perpendicular to the pipe axis.
Upon ignition of the explosive, the explosion shock wave reflects off the opposing V surfaces of the grooved wheel to focus onto the respective metal liners. The opposing liners are driven together into a collision that produces a fluidized mass of liner material. Under the propellant influence of the high impingement pressure, this fluidized mass of liner material flows lineally and radially along the apex plane at velocities in the order of 22,000 ft/sec, for example. Resultant impingement pressures against the surrounding pipe wall may be as high as 6 to 7×106 psi thereby locally fluidizing the pipe wall material.
This principle may be applied to large diameter pipe such as well casing which may be cut while positioned within a wellbore with a toroidal circle of explosive having an outside face formed in the signatory V-groove cross-section. This toroidal circle of explosive is placed and detonated within a toroidal cavity of a housing. However, formation of an explosive torroid of sufficient size to sever a large diameter casing requires relatively large quantities of explosive. As an integral unit, such quantities of explosive exceed prudent transportation limitations. For practical reasons of transport and safety, therefore, the mass of the toroidal explosive circle is divided into multiple, small quantity modules of cross-sectional increments which are transported to a well site in separate, isolated packages.
Explosively cutting a 20 inch casing may require a shaped charge of as much as 1000 gms. (35.27 ounces) of high explosive (ex. HMX). However, international standards of transportation safety (United Nations Recommendations on the Transport of Dangerous Goods, Edition 17, Vol. I, Chapter 2.1, Division 1.4) limit the public transport of a single unit of hazard class or high explosive to 45 gm (1.59 ounces). Consequently, to transport a shaped charge cutter of size sufficient to cut a 20 inch casing, it is essential for the explosive elements of the cutter to be designed for shipment as a multiplicity of small, less than 38 gm./unit, modules configured for operational assembly at the point of use.
Unfortunately, the environmental circumstances of a drilling rig floor, which is where final cutter assembly must occur, are often severe and usually not conducive to the attentive care required for final assembly of a high explosive tool. Hence, there are strong incentives to design the individual explosive modules with the greatest degree of assembly ease and tolerance. But large module assembly tolerance often results in collective space between modules. In the case of modular assembly for shaped charges, such assembly space can severely diminish the cutter capability.
Other issues for large diameter casing cutters arise with deep wells under considerable hydrostatic pressure. Large surface areas for prior art casing cutter housings may be distorted under deep well fluid pressure, also resulting in reduced cutting capacity or a malfunction of the tool.